Generally used more in order to parody or caracature a Scots-English accent than it is used in real usage by Scottish people. Particularly when combined with the suffix expression "the noo".

Technically, it means "oh yes". But generally with an emphatic edge which would place it closer in meaning to "of course". Elevated pronounciation would infer the added in meaning of "isn't it/that obvious?".

The phrase itself is heavily susceptable to region variation of exact pronounciation.